1. Field of the Invention
This invention relates to a semiconductor laser, and more particularly to a semiconductor laser in which a substrate and/or a semiconductor layer is transparent to the oscillation wavelength.
2. Description of the Related Art
Recently semiconductor lasers have come to be widely used in light communication, optical discs, laser printers and the like. Advent of high output semiconductor lasers not lower than 1W has realized application of semiconductor lasers to sublimation of dye by use of a laser spot as a high density heat source or to image forming, processing of material or soldering by use of laser abrasion.
At present, there has been employed a 0.98 .mu.m-band semiconductor laser having an InGaAs strained quantum-well as a pumping light source or a heat source for a fiber amplifier in light communication. Further an InGaAs strained quantum-well semiconductor laser having an oscillation wavelength in the range of 0.9 to 1.1 .mu.m is important as a light source for generating a second harmonic laser beam of blue or green. However it has been found that it is difficult for the oscillation spectrum to be of a single mode in the conventional InGaAs strained quantum-well semiconductor lasers. This is because the gain spectrum is modulated by a multiple resonator effect based on interference by reflected light from n- or p-side, mainly from the electrode surface.
Similarly in a short wavelength semiconductor laser which has an InGaN active layer and is expected as a light source for a high-density optical disc, periodic modulation in the gain spectrum also appears. See "The Blue Laser Diode, GaN Based Light Emitters and Lasers", Springer, Berlin 1997, Chap. 13, 14. This is also because the gain spectrum is modulated by a multiple resonator effect based on interference by reflected light from n- or p-side, mainly from the electrode surface. Though the semiconductor lasers of such material are now under development and accordingly their characteristics are not clear in detail, they are unstable in the oscillation spectrum and can generate noise due to multiple mode oscillation and mode hop when used as a light source for optical discs or very fine printers.
Accordingly there has been a demand for suppressing the multiple resonator effect in the semiconductor laser of the type described above so that the semiconductor laser can stably oscillate in a single line spectrum.
Generally a semiconductor laser in a short wavelength range is formed by superposing a semiconductor layer on a substrate and/or a buffer layer which acts as an absorption medium on oscillating light. Further a cap layer on which an ohmic electrode is formed often acts as an absorption medium on oscillating light. Such a semiconductor laser includes, for instance, an AlGaAs laser having a GaAs substrate and an InGaAlP laser and in such semiconductor lasers, a relatively stable oscillation in a single line spectrum can be obtained when transverse mode control is carried out by an optical waveguide structure. On the other hand, in an InGaAs quantum-well semiconductor laser having a GaAs substrate, oscillation spectrum has a plurality of peaks. This condition is shown in FIGS. 10A and 10B. FIGS. 10A and 10B show the characteristics of an InGaAs quantum-well semiconductor laser having a GaAs substrate which is for pumping a fiber amplifier and oscillates near 975 nm. FIG. 10A shows the emission spectrum in a state below the threshold condition and FIG. 10B shows the emission spectrum in a lasing state. As shown in FIG. 10A, an emission spectrum modulated at a period longer than Fabry-Perot modes appears from a state below the threshold condition, and the oscillation spectrum is in a multiple mode reflecting the periodically modulated gain spectrum as shown in FIG. 10B. Though this semiconductor laser can oscillate in a single longitudinal mode in a limited light output range and a limited temperature range, the longitudinal mode varies with change in the light output and the temperature and noise is generated. Further though the oscillation wavelength can be stabilized by use of a grating, a band pass filter, a fiber grating or the like, it is difficult to stably lock the wavelength due to its long periodic structure, and the oscillation wavelength sometimes hops over the adjacent mode to a mode spaced from the original mode by several modes according to modulation at a long period. Thus it is very difficult to stabilize the oscillation wavelength.
In an InGaAs quantum-well semiconductor laser, a substrate, a buffer layer for a high quality crystal growth and a cap layer on which an ohmic electrode is formed are generally formed of GaAs. Since the forbidden band of the GaAs is transparent to the oscillation wavelength of the InGaAs, a part of the oscillating light is reflected at the surface of the substrate and/or the surface of the electrode in contact with the cap layer and the spectrum is modulated by multiple interference.
The influence of the interference is more significant in a multiple transverse mode laser where the stripe width is large. We have found that there is a large difference in spectrum between the case where the substrate and the like are transparent to the oscillation wavelength and the case where the substrate and the like are untransparent to the oscillation wavelength also in a gain optical waveguide type multiple transverse mode laser having an oxide film stripe 50 .mu.m wide. That is, we have found that in a semiconductor laser which oscillates near 800 nm to which the GaAs substrate is untransparent, the oscillation spectrum has a single peak as shown in FIG. 11 while in a semiconductor laser which oscillates near 1070 nm to which the GaAs substrate is transparent, the oscillation spectrum has a plurality of peaks as shown in FIG. 12. Further we made an InGaAs active layer semiconductor laser having a GaAs substrate and confirmed that in 0.94 to 1.07 .mu.m band lasers, the oscillation spectrum could not have a single peak. When such a multiple peak oscillation spectrum is employed to pump fiber amplifiers or solid state lasers, there arise problems that pumping efficiency deteriorates due to a large width of spectrum relative to the absorption band of the medium to be pumped and partition noise is generated due to switching between different wavelengths. Further when such a multiple peak oscillation spectrum is employed as a heat source for image forming, for instance, in a printer, image quality is deteriorated due to fluctuation in intensity.
Similarly in a short wavelength semiconductor laser having an active layer of InGaN, a GaN or AlGaN layer as a buffer layer and a GaN layer as a cap layer are transparent to the oscillation wavelength and a sapphire or spinel as a substrate is transparent to the oscillation wavelength. Accordingly the spectrum of the semiconductor laser is modulated by the multiple resonator effect. In a commercially available InGaN blue LED, the emission spectrum is also modulated by the similar interference effect. This was confirmed by observing the emission spectrum from the LED tip end face with the resin on the end face removed. Thus it has been found that when the material of the substrate and the like is transparent, the emission spectrum is greatly affected by interference. Since a LED is not an oscillator, the effect does not lead to instability in a LED. However in the case of a semiconductor laser, the effect makes the oscillation mode instable and leads to increase in noise.